1. Field of the Invention
The present invention relates generally to data storage systems for use with computers and more particularly to an apparatus and method for transferring the information from one or more disks to a backup tape and restoring the information from the tape back to the disk.
2. Description of the Prior Art
Data processing systems commonly use one or more hard disks for storage of data or programs that are not currently required to be in the internal random access memory of the host computer. The host will retrieve from the disk system the particular information currently required and will send to the disk system for storage new or updated information or data or programs which the host does not require at that time. Modern disk systems are capable of rapidly storing and retrieving large quantities of data and are, in general, highly reliable devices. However, occasionally failures or faults do occur in disk systems and, in the event of such a failure, all or part of the data stored on the system may be lost or rendered unreliable.
It is, therefore, highly desirable from time to time to make a copy of the information stored on the disk system so that the user of the system is able to reconstruct the data into a known condition in the event of a system failure or other problem causing a loss of data integrity. A common practice with larger computer systems is to periodically perform a backup of the data on the disk system. Typically this is done by copying the data, or selected portions of it, onto one or more magnetic tapes. The tapes are then retained for a period of time, as determined by the system user, and are available for use in restoring the system to a known condition, if necessary. In some cases it may be necessary to restore the entire disk file system while in other cases, such as inadvertant deletion of a few files by an individual user, it may be necessary to restore only those particular files.
The backup needs to be a "snapshot" of the disk systems at a particular instant in time. Since modern data processing systems often have disk systems with extremely large storage capacity, the backup operation can sometimes take several hours to transfer the disk data to tape. Once the backup operation has started, some procedure must be used to ensure that the data on the disk is not modified until it has been stored on tape. If the users of the system are allowed unrestricted access to the data files on the disk, the exact condition of the data on the disk would not be consistent. The typical means of ensuring data consistency is to restrict users from having access to the data during the backup operation. Many times these backups are performed during late night hours when users are usually not inconvenienced by being locked out of the system. However, as data processing systems become larger and as networked users around the world become able to access the same system 24 hours a day, it becomes much less acceptable to prohibit access to the system for prolonged periods of time.
The present invention resolves this problem by allowing users unrestricted access to the system while the backup operation is underway.